Eduardo - I had a second so I did a quick check. Looks like you need to convert to Base64 when submitting to the api... so there's probably some testing and research you'll need to do.
I haven't dealt with this myself - but this is where I'd start....
My issue is with attachments as well, but it's not that I want to send one. The Gmail channel makes the Attachments URL a required field for some reason. Would love to turn this off, if anyone has experienced it.
In case you never figured out the ASCII issue, using HTML in the body solves this. I use this to send an email to an address from an email field on my table and use markup around the field placeholder in the body.